CHANGES IN AGGREGATION OF PROTOPLASM 63 



in the endoplasm), and also round the cell nucleus. 

 It will be noted that all these phenomena of the living 

 cell are strictly in accordance with what we learned of 

 the behaviour of non-living colloids in the last chapter. 

 The special characters of protoplasm are largely 

 dependent on the fact that it has the power of readily 

 passing from the sol to the gel condition and back again. 

 Normally it may be said to exist somewhere near the 

 margin of the sol and the gel conditions. About o C. it 

 gels and becomes inactive, though as we shall see in 

 later chapters it is not " killed," i.e. its essential struc- 

 ture is not destroyed, at far lower temperatures. As 

 the temperature rises it becomes more labile and more 

 active, till above 40 C. changes begin to occur which 

 eventually destroy its structure altogether. Many other 

 causes besides changes of temperature and the loss or 

 absorption of water send the protoplasm from the sol to 

 the gel condition and back again, and it is on the changes 

 in aggregation of the protein molecules and groups of 

 molecules, the association and dissociation of these with 

 one another, and with molecules of fats and salts, that 

 what we call the vital activities of the protoplasm 

 depend. The more aggregation occurs, the more gel- 

 like and relatively stable the structure of the protoplasm 

 becomes, the less free the motion of particles and 

 molecules from one part of the living cell to another. 

 Consequently different chemical and physical processes 

 can go on in different parts of the cell (as is often 

 the case) without interfering with one another. Dis- 

 aggregation leads to the opposite effect. Many of the 

 more specialised animal cells have their protoplasm 

 more or less permanently in the gel condition, and the 

 specialised character of their activities depends largely 

 on this fact. But on the whole the life activities 



